1. Field of the Invention
The present invention relates to communication systems. More specifically, the present invention relates to high speed word generators used in communication systems.
While the invention is described herein with reference to illustrative embodiments for a particular application, it is understood that the invention is not limited thereto. Those of ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof.
2. Description of the Related Art
Devices that generate random words are widely used in communication systems. For example, in secure systems, it is desirable to protect a channel of communication from unauthorized eavesdropping. In such systems, the data may be encoded to make the transmission appear as random noise. In this application, a uniform random word generator would be useful in generating random words of a predetermined length which would fill the gaps between the data words. If the word generator is not random, but instead has patterns, it will generate spectral spurs. That is, a spectral analysis of the signal will reveal the pattern and compromise the effectiveness of the encryption scheme.
Another application for random word generators is in spread spectrum communication systems. In this case, the phase of a carrier signal is randomly modulated so that the signal is spread over a limited bandwidth and appears as noise. Again, if the random word generator has patterns, those patterns will show up on spectral analysis.
Yet another application for random word generators is found in frequency hopping systems where, for example, a random word generator may be used to change the frequency of a carrier signal. Once again, if the generator is not random, the resulting pattern may be recognized allowing an eavesdropper to lock onto the signal.
For these applications, random word (or number) generators have been used. Random word generators are generally built from random noise generators. Resistors and diodes generate noise randomly and are therefore often used for this purpose. When a dc voltage is applied to the random noise generator, the output voltage goes up and down randomly about an operating point. To generate a digital output, the simple random word generator typically includes a comparator which digitizes the output of the noise generator.
While many variations on the simple random word generator are known, most schemes suffer from dc drift. That is, the operating point of the system may shift, due to thermal or other effects, and change the output characteristic from a preferred Gaussian distribution to one favoring one state or the other. Attempts to address the problem by using ac amplifiers have been less than totally successful as the ac systems have an inherent bandwidth limitation outside of which, patterns begin to appear.
Pseudo-random generators are generators which are less than random and have some pattern or repetition rate. Pseudo-random generators with sufficiently long patterns or low repetition rates have been used with limited success for some applications. However, such generators are typically more complex and therefore costly than a random generator of comparable performance capability.
Thus, there is a need in the art for a simple high speed generator of N-bit random words with uniform probability distributions.